U.S. Pat. No. 3,379,000 describes the manufacture of stainless steel fibers by bundled drawing, i.e. starting with a bundle of wires embedded in a metal matrix which differs from the wire metal, e.g. in copper sheaths. After the drawing, the copper is stripped in a nitric acid solution. The fibers which are obtained using this patented method still show some traces or remnants of the matrix material (copper) at their surfaces.
To turn the stripping of the matrix metal in HNO.sub.3 into an ecologically sound process, considerable sums must be spent on the neutralizing of the generated nitrogen oxide fumes and on converting the used stripping fluid into disposable waste. Apart from that, some remnants of the metal matrix are left on the fiber surface. Thus, this surface is somewhat contaminated and this can be a disadvantage in certain applications.
With the present invention it is now possible to manufacture metal fibers, using the bundled drawing method as described above and yet avoid this contamination of the fiber surface.
The average concentration of matrix metal in the surface layers of the fibers thus obtained is at the most 0.2% at. The average copper content in the surface layers of standard metal fibers, obtained by applying the HNO.sub.3 stripping process to a copper matrix, is more than 2% at. The thickness of the surface layer under consideration is about 50 .ANG..
The metal fibers obtained by applying the present invention may be stainless steel fibers with a chromium content of at least 10% by weight. Specifically, the fibers will contain at least 16% Cr and also Ni. Furthermore, the invention can be used to manufacture refractory fibers containing Fe, Cr, Al and, optionally, Y or rare earths (as is described, for example, in U.S. Pat. No. 4,139,376) and fibers from Ni/Cr alloys, Hastelloy.RTM., Inconel.RTM., titanium or Carpenter.RTM.20cb3.
It is also an object of the invention to provide stainless steel fibers of the kind specified above and having a reduced average Chromium content (a lower Cr/Cr+Fe+Ni-ratio) at their surface, i.e. with a Cr/Cr+Fe+Ni-ratio between 1% and 15% wherein the Cr, Ni and Fe-contents are expressed in at %. Even if said fibers retain more than 0.2% at of matrix metal at their surface, the lower Cr-content offers the advantage of a better corrosion resistance as will be explained further on.
The invention also comprehends a process and apparatus for the continuous electrolytic removal of the matrix material from a drawn composite bundle. Thereby, the bundle serves as an anode and the embedded bundle is transported continuously through successive electrolytic baths at a temperature of over 20.degree. C.
It is a further object of the invention to provide a discontinuous or batch process for electrolytic removal of the matrix material from a drawn composite fiber bundle. This process is particularly useful when thin bundles have to be treated which can hardly sustain throughput forces in a long continuous stripping installation.
Contrary to the process in conventional continuous electrolytic stripping installations, the bundle does not make contact with current carrying (anodically connected) contact elements. Cathodic transition cells are present between said baths. During the process, the bundle is supported at the level or in the vicinity of these transition cells. The arrangement and the distances between the various cells or baths are such that in the spaces between the electrolytic baths and the cathodic transition cells the current is conducted by the bundle. During the process, at least part of the matrix material is deposited on the cathodes facing the bundle in the electrolytic baths. All these measures contribute to the development of a more economical process with the additional advantage of a higher quality fiber product. The fibers are less damaged as will be shown further and some of their characteristics are more constant, i.e. display less variation than in case of standard bundled drawn fibers.